There is no real right or wrong answer to this and it is of course dependant on roof material used.
For both felt and shingles in is better to apply the insulation (flexible foil type) to the outside of the roof, then a flexible ply sheet fixed to the roof boards and the final roof covering to that.
If you have source yourself corrugated metal roof sheet it is best to fix felt to the roof as standard, then batten the roof, insulation between the battens and fixed and then the metal roof.
However, you can also batten the roof and apply the insulation, then a flexible ply with felt on top and then the metal roof.
Alternatively you could also insulate under the roof but you then run the risk of condensation between the roof and insulation, you would also remove the attractive hoops in the roof with this method.
Answers to most questions can also be found by typing into the search bar in the widget or our info center or please contact us to help you further with any questions you have.